| The impact of NOx emissions on human and environment becomes more and more serious with rapidly increasing use of diesel engines. The Urea-SCR system of diesel is a major post-processing device that can reduce the NOx emission and meet Europe IV or above emission regulations. To meet the more and more restrict requirements of energy saving and environmental protection, the Urea-SCR system must achieve the object of high efficient transformation and low pressure loss, and avoid the secondary pollution problem brought by spraying ammonia too much.This research is based on the863Project-The Research and Development with Environmental Protection and Efficient Diesel Combustion System of The New Generation Cars and Post Processing System, whose number is2008AA11A116,and the science and technology innovation supported program-The Research on Urea Spray and Mixing Rule Of the Diesel Engine Urea-SCR System. The spray atomization, pressure loss, the reaction characteristics and ammonia injection MAP of diesel SCR system is studied and analyzed by combining the numerical simulation and experiment verification. The major research work and innovations are as follows:1. Based on the pressure swirl jet model and the discrete phase atomizer mixing model, the influence of nozzle spray distance, spray position and orientation, number and diameter of spray orifice on the concentration distribution of fluid drop at the entrance of the catalyst carrier were studied by numerical simulation. Using four orifices,0.5mm aperture, nozzle in the pipe center and radial spray can bring out the best result of spray atomization. With bench test, the NOx conversion rate can be improved to95percent or more, which verifies the rationality of the simulation results.2. A porous medium model for SCR system is set up. The effects of expansion cone angles, volume and length, carrier and layer thickness of the catalytic reactor on the pressure loss at the free flow region of catalytic reactor are investigated by using standard κ-ε turbulence model.3. In order to improve the atomization mixing of urea aqueous solution, three different structures of the mixer are designed. Numerical simulation analysis is conducted for two kinds of typical working condition. A corresponding uniformity indicator is proposed from simulation and experiment results. Combining the uniformity indicator and the pressure loss, the optimization result is obtained by comparing the consistency of simulation with experiment.4. The research is studied the chemical reaction mechanism and reaction kinetics model of the diesels SCR system and set up the model that based on transient absorption desorption reaction. The main content include:the influence of temperature and air flow rate for the storage and storage release rate of NH3; the effects of NO2, O2and H2O in the exhaust for the conversion efficiency of NOx in the SCR system.5. The research Analyzed the inflatable coefficient and the initial MAP of the fuel injection pulse width. The control strategy is drawn up based on open-loop control strategy combined with the emissions MAP, exhaust temperature MAP, exhaust back pressure MAP and NOx conversion rate MAP. Studied the NOx generation rate and injection rate of urea solution Combined with experiment, It made a correction for the temperature of urea solution, exhaust temperature, the ability to store ammonia of catalyst, ammonia nitrogen ratio and some influential factors.The research is studied the spray atomization, catalytic conversion and control strategy of the diesel urea-SCR system in details and depth. The results can provide design basis and theoretical guidance for realistic diesel urea-SCR system. |
PDF Full Text Request